Possible to control text color in the text editor?
So @David's comment I think points you in the right direction, but maybe it can be made a little simpler.

Basically you can give individual characters (or words, or sentences, etc.) different materials (i.e. material indexes).  Each material can, obviously, vary not only in color but any attribute you can put into a material.

So, with the text object below, in edit mode I highlight the characters I wish to give a new material.  



You can add a new material slot if you need one, create a new material if you need one, etc., but at the end of the day all you need to do to assign the selected text to the chosen material slot is to click the "Assign" button.

You've then got text with different materials!